Respiratory system pressure-volume (P-V) studies, 67Ga-citrate scans, and chest radiographs were made in the corticosteroid-treated rat model of Pneumocystis carinii pneumonia. The steroid treatment used to provoke Pneumocystis infection in this model resulted in a reduction in body weight, lung weight and lung volumes compared to age-matched controls but no change in the normalized pressure-volume curve. P. carinii infection was associated with increased lung wet weight and flattening of the respiratory system P-V curve when compared to either age matched controls or steroid-treated animals on trimethoprim-sulfamethoxazole prophylaxis for P. carinii. Radiographs were interpreted as positive in only three of 11 animals with P. carinii, whereas 10 of 12 animals showed positive gallium-67 lung scans. We conclude that both gallium uptake and altered lung mechanics occur in the rat with Pneumocystis carinii and may reflect increased alveolar permeability and surfactant abnormalities noted in other studies.